Funani, an African river hippo at the San Diego zoo, gives her female calf, Jazi, a little nudge as they swim. The hippopotamus – Greek for "river horse" – is a semi-acquatic mammal that spends much of its time keeping cool in the water. The creatures appear more graceful and even elegant when fully submerged, but their enormous size means an adult hippo can often get by just by walking or standing on a lake or river bed. Photograph: Ken Bohn/AFP/Getty Images